Terra firme

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geography of Brazil

...again as the watercourse approaches the Atlantic, but no delta extends into the ocean. The basin’s most widespread topographical features are gently undulating hills called
terra firme (“solid ground”), composed of layers of alluvial soil that were deposited as much as 2.5 million years ago and subsequently uplifted to positions above flood...

...are subject to annual flooding, with consequent soil enrichment; however, most of the vast basin consists of upland, well above the inundations and known as
terra firme. More than two-thirds of the basin is covered by an immense rainforest, which grades into dry forest and savanna on the higher northern and southern margins and into montane...

Amazonian soils are also leached but not as deeply. In the
terra firme of the rainforest, dead organic matter quickly decays and is recycled. However, once the overlying forest canopy is destroyed—e.g., by clear-cutting or burning—that regenerative cycle is interrupted, and many nutrients and organic matter are lost. More fertile Amazonian...